The present invention relates to improvements in an automatic focusing apparatus which is intended to hold constant the distances of the focusing etc. of a projection aligner for use in the production of semiconductor devices.
As a projection aligner for producing a semiconductor device of fine pattern dimensions, there is a 1/10 reduction projection aligner in which a magnified pattern of 10 times is reduced and projected on a substrate through a reduction lens of high resolution while step-and-repeat operations are being performed.
The focusing depth of the reduction lens of high resolution is as very small as .+-.1 to 2 .mu.m. In addition, in order to hold an image distortion within about 0.1 .mu.m/10 mm-square, the surface of the substrate under the projection needs to be always held at a precision of .+-.0.5 .mu.m or so with respect to the focal position of the reduction lens.
As apparatuses for focusing reduction lenses, there are ones disclosed in Japanese Laid-open Patent Application No. 52-119927 and Japanese Laid-open Utility Model Registration Application No. 56-12201.
FIG. 1 shows the arrangement of such prior-art focusing apparatus. Clean air under a constant pressure is sent from a pressure regulator 409 to a detector 402, disposed at the lower end of a lens housing 401, and to a reference detector 406 by pipes 404 and 408 respectively, and it is spurted from the orifices of the detectors. Pressure signals from the detector 402 and the reference detector 406, corresponding to the respective distances of the orifices to a substrate 412 and a reference plate 418, are sent to a pressure transducer 421 by pipes 403 and 407 respectively. An electric signal 422 from the pressure transducer 421 is applied via an amplifier 423 to a servomotor 424, which drives a base 413 so that the electric signal may become null. Thus, the clearance between the substrate 412 and the detector 402 is held at a predetermined optimum focal length at all times. In setting the optimum focal position, the fact is exploited that when the clearance between the reference detector 406 and the reference plate 418 is varied by means of a screw 417, the clearance between the detector 402 and the substrate 412 changes accordingly.
Here, the base 413 is mounted on a rigid base 419 through a transfer means 414.
On the other hand, in case of actually using the projection aligner, it is often required to vary a focal position on a substrate stepwise over a certain range above and below an optimum focal position and then move the substrate finely, thereby to project a pattern on the substrate in an offset position. In the above example of the prior art, this requirement is dealt with by varying the clearance between the reference detector 406 and the reference plate 418. This measure, however, has the following four disadvantages: (1) Since a great distance between the reference detector 406 and the detector 402 degrades servo characteristics, the reference detector 406 cannot be disposed far, and hence, the remote control is difficult. (2) The reference detector 406 for setting the optimum focal position to serve as a standard is made variable, so that the standard set position is liable to move. (3) The reference detector 406 is made movable, so that the centralized remote control employing a computer is difficult. (4) With the prior-art arrangement, that positioning accuracy of the substrate 412 which is attained by shifting the position is .+-.0.5 .mu.m or so.